1. Field of the Invention
This invention relates to a brushless motor drive circuit and, more particularly, to an improvement of the temperature dependency of the torque ripple correction capability of a brushless motor drive circuit.
2. Related Background
The inventor of the present invention has filed a patent application for a brushless motor drive circuit capable of suppressing reduction in the level of synthetic torque and at the same time having a high torque ripple correction capability. This application is U.S. patent application Ser. No. 547,798, now U.S. Pat. No. 5,173,645. FIG. 3 of the accompanying drawings diagrammatically shows a circuit according to the above cited invention.
Referring to FIG. 3, a position detecting means comprising Hall devices Hu, Hv and Hw is supplied with power from a power source Vcc. The Hall devices Hu, Hv and Hw detect the rotary position of a rotor comprising a rotor magnet (not shown) having a 2.times.n magnetized poles. The Hall devices selectively generate three sinusoidal wave signals Vu, Vv and Vw, whose phases are differentiated from one another by 120.degree. as shown in FIG. 4(a), depending on the rotary position of the rotor relative to a stator. The stator comprises three drive coils Lu, Lv and Lw arranged for 3-phase configuration.
An amplifying and synthesizing circuit 44 comprises transistors Q1 through Q6, variable current sources CS6 through CS8 for generating an electric current Io, resistors R3 through R5 and diodes D1 and D2. The variable current sources CS6 through CS8 modify the input voltage applied from an adjuster terminal T1 on a variable basis to provide an electric current Io. The amplifying and synthesizing circuit 44 amplifies the output signals Vu, Vv and Vw of the Hall devices Hu, Hv and Hw and logarithmically compresses them so that the signals have their respective waveforms reduced flat at and near the inflection points. The signals, thus, become somewhat rectangular, pulse-like signals and are synthesized to produce three phase-differentiated soft switching signals Vu', Vv' and Vw' having a waveform as shown in FIG. 4(b). Differently stated, the output signals Vu, Vv and Vw of the Hall devices Hu, Hv, Hw are simplified by the transistors Q1 through Q6 and the collector outputs of the transistors Q1 through Q4 are synthesized to become a soft switching signal Vu' while the collector outputs of the transistors Q3 through Q6 are synthetically processed to produce a soft switching signal Vv' and those of the transistors Q2 and Q5 are synthesized into another soft switching signal Vw'.
The soft switching signals Vu', Vv' and Vw' from the amplifying and synthesizing circuit 44 then pass through respective resistors R7 through R12 and are converted into electric currents by a 3-differential amplifier comprising PNP-type transistors Q31 through Q33, NPN-type transistors Q34 through Q36 and variable current sources CS9 and CS10. The electric currents are amplified by the same 3-differential amplifier. The output currents Iu1, Iv1 and Iw1 as well as Iu2, Iv2 and Iw2 of the 3-differential amplifier are applied to a predriver PD by way of a mirror circuit comprising transistors Q37 through Q42 and Q43 through Q48. Then, for example, at phase U, the soft switching signal Vu' from the amplifying and synthesizing circuit 44 passes through the resistors R7 and R10 and is converted to electric currents by the transistors Q31 and Q34, which amplify the currents, the output current of the collector of the transistor Q31 being fed back to the base (point U2) of the transistor Q34 by a mirror circuit constituted by transistors Q43 and Q52 and resistors R17 and R20. The output current of the collector of the transistor Q34 is, on the other hand, fed back to the base (point U1) of the transistor Q34 by a mirror circuit constituted by transistors Q37 and Q49 and resistors R13 and R16. The level of the currents fed back to the bases of the transistors Q34 and Q31 is held significantly lower than that of the current Io from the variable current sources CS6, CS7 and CS8.
Similarly at phases V and W, the soft switching signals Vv' and Vw' from the amplifying and synthesizing circuit 44 respectively pass through the resistors R8, R11 and R9, R12 and are converted to electric currents by the transistors Q32, Q35 and Q33, Q36, which amplify the currents. The output currents of the collectors of the transistors Q32 and Q33 are respectively fed back to the bases (points V2 and W2) of the transistors Q35 and Q36 by mirror circuits respectively constituted by transistors Q44, Q53 and Q45, Q54 and resistors R18, R19 and R20. The output currents of the collectors of the transistors Q35, Q36 are, on the other hand, fed back to the bases (points V1 and W1) of the transistors Q32 and Q33 by mirror circuits respectively constituted by transistors Q38, Q50 and Q39, Q51 and resistors R14, R15 and R16. The level of the currents fed back to the bases of the transistors Q32, Q33, Q35 and Q36 is held significantly lower than that of the current Io from the variable current sources CS6, CS7 and CS8.
With an arrangement as described above, voltages Vsu1, Vsv1 and Vsw1 respectively between the cathode s of the diode D2 and the points u2, v2 and w2 and voltages Vsu2, Vsv2 and Vsw2 respectively between the cathode s of the diode s and the points u1, v2 and w2 as shown in FIG. 5. The combined diodes D6 and D9, D7 and D10 and D8 and D11 operate as amplitude limiters for limiting the amplitudes of voltages Vsu1, Vsv1, Vsw1, Vsu2, Vsv2 and Vsw2.
Upon receiving an output signal from the predriver PD, a group of transistors Q55, Q56 and Q57 operates to cause source currents .alpha.Iu1, .alpha.Iv1 and .alpha.Iw1 to flow into the respective drive coils Lu, Lv and Lw. Another group of transistors Q58, Q59 and Q60 operates to cause sink currents .alpha.Iu2, .alpha.Iv2 and .alpha.Iw2 to flow out of the respective drive coils Lu, Lv and Lw. Source currents .alpha.Iu1, eIv1 and .alpha.Iw1 are obtained by multiplying by .alpha. the respective input currents Iu1, Iv1 and Iw1 of the predriver PD by means of predriver PD and the group of transistors Q55, Q56 and Q57. Sink currents .alpha.Iu2, .alpha.Iv2 and .alpha.Iw2 are obtained by multiplying, by .alpha. the respective input, currents Iu2, Ic2 and Iw2 of the predriver PD by means of predriver PD and the group of transistors Q58, Q59 and Q60.
Electric current detecting resistor Rs detects any electric currents flowing through the drive coil Lu, Lv and Lw and converts them to voltages. A voltage developed from resistor Rs is compared with a motor speed control signal Vctl at an electric current feedback amplifier Ai and an error voltage representing the difference, if any, between them is utilized to modify the variable current sources CS9 and CS10 and to control the electric currents Ictl coming from them. Thus, the current ICtl is so controlled that the current flowing through the resistor Rs is kept constant so long as the control signal Vctl is kept constant and a constant electric current is supplied to the drive coils Lu, Lv and Lw.
FIG. 6 schematically illustrates the waveforms of the source current .alpha.Iu1 and the sink current .alpha.Iu2 for the U-phase drive coil Lu determined by a simulating operation. It is to be noted that the source current .alpha.Iu1 and the sink current .alpha.Iu2 flow through the current detecting resistor as reactive currents during current conductive periods To when no current flows through the U-phase drive coil Lu as the U-phase transistors Q55 and Q58 are on simultaneously; in contrast, they are never on simultaneously during current conductive periods a when a current flows through the U-phase drive coil Lu. A similar statement applies to V- and W- phases. The level of torque ripple of a brushless motor drive circuit having a configuration as described above is approximately 6% when determined by simulating drive currents for the drive coils Lu, Lv and Lw and other factors. A remarkable improvement is, therefore, achieved by such a drive circuit.